Multi-port devices are characterized by their number of ports, typically referred to throughout this application as N, where N is 2 or greater. S-parameter measurement instruments, such as a vector network analyzer (VNA), are used to characterize such a multi-port (i.e., N-port) device under test (DUT, often simply referred to as a “device”) by measuring complex transmission and reflection characteristics of the DUT.
In the RF and microwave regions virtually all devices are characterized by their S (or scattering) matrices. The S matrix is composed of S-parameters. S-parameter measurement is the standard method for device characterization over a very wide range of frequencies, from less than 1 MHz to above 40 GHz. These parameters are used because they are easily determined, they provide directly relevant measures of device performance, and they are well defined for various types of devices. If other device representations are required, such as impedance or admittance parameters, then these can be readily deduced from the measured S-parameters.
More specifically, S-parameters of a multi-port device characterize how the device interacts with signals presented to the various ports of the device. An exemplary S-parameter is “S12.” The first subscript number is the port that the signal is leaving, while the second is the port that the signal is being injected into. S12, therefore, is the signal leaving port 1 relative to the signal being injected into port 2. The four S-parameters associated with an exemplary two-port device are:                S11 is referred to as the “forward reflection” coefficient, which is the signal leaving port 1 relative to the signal being injected into port 1;        S21 is referred to as the “forward transmission” coefficient, which is the signal leaving port 2 relative to the signal being injected into port 1;        S22 is referred to as the “reverse reflection” coefficient, which is the signal leaving port 2 relative to the signal being injected into port 2; and        S12 is referred to as the “reverse transmission” coefficient, which is the signal leaving port 1 relative to the signal being injected into port 2.        
A large number of commercial test systems are available for S-parameter measurement. Such systems are generally referred to as network analyzers. These instruments typically fall into two classes: scalar and vector. Scalar analyzers determine the amplitudes of the S-parameters only, whereas vector analyzers (VNAs) determine both the amplitudes and the phases. Scalar analyzers are far less flexible and far less accurate than vector analyzers, and are typically only employed in low-grade applications where equipment cost is a driving factor. Although embodiments of the present invention are generally applicable to VNA test instruments, the embodiments may also apply to other types of instruments that characterize S-parameters (or other equivalent measurements) for a multi-port DUT.
Commercial VNA systems typically include a signal generator and a combination of splitters and directional couplers that connect the measurement ports of the VNA to its amplitude and phase detection circuitry (samplers). A typical DUT to be characterized by such a VNA has one, two or more ports, typically with coaxial or waveguide interfaces. For an N-port DUT, the S matrix (N×N) can be defined by: b=Sa, where a is an N-component vector containing the amplitudes of the waves incident on the device ports, and b is a vector containing the amplitudes of the outgoing waves. More formally, the wave amplitudes are defined by: ai=(Vi+ZiIi), 2; and bi=(Vi−ZiIi)/2, where ai is the incident voltage wave amplitude, bi is the outgoing voltage wave amplitude, Vi is the voltage, Ii is the input current, and Zi is the normalizing impedance, all for the ith port under test.
The port-normalizing impedances (Zi) are typically chosen to be equal to the characteristic impedances of the coaxial cables in the test system, which are 50Ω in most cases. If a given port is terminated with its normalizing impedance (a matched load) then the incident wave amplitude at that port is identically zero (from ai=(Vi+ZiIi)/2).
When a DUT is connected to the test ports of a network analyzer, a signal is applied to each device port in succession, and the reflected and transmitted waves are detected with the aid of the directional couplers. The S-parameters for the DUT are then deduced by measuring the amplitude and phase of each of these waves relative to those of the input signal.
In practice, there are inevitable hardware imperfections or errors in any VNA test system, which are principally related to port mismatch, coupler directivity, and instrument frequency response. Without correction, these imperfections can produce significant measurement errors. Such imperfections are typically compensated for though appropriate VNA calibrations. VNA calibrations are typically performed by connecting physical standards (also known as mechanical primary standards) to each of the ports of the VNA for the purpose of calibration. Electrical characteristics of the standards are derived from known physical properties of the standards, such as physical dimension, conductor material, and the like. The errors of the VNA are typically determined by computing the difference between the VNA measured response of the standards and known electrical characteristics of the standards. After the VNA is calibrated, an uncharacterized DUT can be connected to the VNA for measurement, and the errors associated with the VNA (determined during calibration) can then be mathematically removed from the measurement of the DUT. Many modern VNAs include internal automatic calibrators that perform the calibration.
Calibrations (typically multi-port calibrations) are normally performed and then applied as a whole. For example, a 4-port calibration is performed and then applied by making 16 S-parameter measurements and using the error coefficients (arrived at during the performance of the calibration) to correct the data. If multiple M-port sub-devices are being tested at once, or a given N-port device is essentially composed of two or more M-port sub-devices (M<N), then either: a) a series of M-port calibrations are performed and recalled sequentially; orb) a full N-port calibration is always used although it may not always be needed. Both of these require more time than a calibration application appropriately scaled to the problem at hand.
The following references, which are incorporated herein by reference, describe exemplary N-port calibrations: R. A. Speciale, “A Generalization of the TSD Network-Analyzer Calibration Procedure, Covering n-Port Scattering-Parameter Measurements, Affected by Leakage Errors,” IEEE Trans. On MTT, vol. 25, December 1977, pp. 1100-1115; and A. Ferrero and F. Sanpietro, “A Simplified Algorithm for Leaky Network Analyzer Calibration,” IEEE. Micr. And Guided Wave Lett.,” vol. 5, April 1995, pp. 119-121.
When VNA measurements were limited to straightforward 1-, 2- and 3-port devices, the application of a calibration was fairly simple, e.g., perform a 1-, 2- or 3-port calibration as appropriate and measure the S-parameters. In the interests of efficiency, some test methods have become compound, e.g., measure two 2-port devices in parallel or measure one while a handler is positioning another. Also, more DUTs have become compound, e.g., two 2-port devices in a single package, or a 1-port and a 3-port device in a single package, etc. Applying the calibrations can be inconvenient in some of these scenarios. Often, more S-parameter measurements are made than required to produce the needed parameters and/or multiple calibration files are sequentially recalled as needed. Both of these arrangements consume excess time and reduce test throughput.
There is a need for more efficient techniques for producing S-parameter measurements and applying VNA calibrations. Preferably, such techniques should increase measurement throughput, particularly for multi-port devices.